For catalyst systems of the type described above, there exist two basic relationships by which much of their chemistry can be explained, i.e., (i) catalyst productivity vs. the aluminum/titanium atomic ratio and (ii) catalyst productivity vs. selectivity (as measured by percent xylene solubles). The catalyst productivity increases precipitously as the aluminum/titanium atomic ratio increases up to 35:1 to 40:1. Above this ratio, productivity increases much more slowly. This region is referred to as the productivity plateau. Normal operating procedure is to add just enough hydrocarbyl aluminum compound to reach the productivity plateau, i.e., 40:1. At this concentration, productivity is maximized and residual catalyst components in the resin are minimized. With regard to selectivity, adding more selectivity control agent causes one to move towards lower xylene solubles and lower catalyst productivity. Adding less selectivity control agent causes one to move towards higher xylene solubles and higher catalyst productivity. As the xylene solubles decrease from higher values toward lower ones, catalyst productivity decreases slowly. As the xylene solubles decrease even more, there is a point at which catalyst productivity falls off dramatically. In this region, control of the reaction is difficult and catalyst productivity very quickly becomes unacceptable. The maximum productivity and minimum xylene solubles (or maximum isotacticity) of a catalyst system, then, is defined by these relationships.
Thus, one of the problems faced by workers in this field is to make improvements in the maximum productivity and isotacticity obtainable for a given catalyst system. Furthermore, it is desirable to increase catalyst productivity over the range of normal xylene solubles operation.